Summary. Our goal is the development and validation of a technology that can measure a wide range of drugs, metabolites, and biomarkers in the blood, urine, and interstitial fluids of living subjects continuously and in real time, an ability that would revolutionize many aspects of biomedical research and healthcare. To achieve this goal we will combine electrochemical aptamer-based sensors (which are reagentless and wash-free and can operate directly in complex samples without fouling) with biocompatible anti-coagulation membranes and advanced drift correction methods to achieve continuous, real-time, multi-day measurements directly in vivo. As preliminary results supporting this goal we demonstrate the continuous, real-time measurement of four different drugs directly in the jugulars of anesthetized rats for periods of more than five hours with 20-second time resolution and sub-micromolar precision. Leveraging these initial results we propose here three further advancements: First, we will expand the platform to the in vivo measurement of nine additional drugs and metabolites, which will demonstrate the platform's versatility. Second, we will improve the platform's stability such that we can perform high-precision in vivo measurements over the course of days, which will enable continuous measurements of long-lived drugs and capture clinically important circadian metabolic variations. Finally, we will expand the platform to the simultaneous measurement of multiple molecules and/or across multiple body compartments (e.g., blood, tissues, brain, bladder), rendering it a powerful new tool for understanding pharmacokinetics and physiology. By creating an unprecedented window into a patient's molecular-level physiological state, the proposed technology would enable many transformative clinical applications, including the high-precision measurement of patient-specific pharmacokinetics, the continuous monitoring of health status via the real-time measurement of specific biomarkers, and, ultimately, feedback- controlled drug delivery.